It is known to provide an L-, or T-, shaped insulated connector for connecting a cable termination, for example, to electrical equipment, such as switchgear for example. At high voltage, say above about 15 kV, and above 24 kV in particular, it is also known to screen such connectors by providing an electrically conductive layer on the outer surface thereof for use with a termination for a screened cable. Such a screened connector is available under the trade name RSTI from Tyco Electronics Raychem GmbH. Screening has the advantages of rendering connectors touch proof and of allowing several connectors, for example one for each phase of a three-phase power supply, to be mounted more closely together, thus reducing the size of the cabinet in which they are contained.
Difficulties have been encountered however, in producing a suitable high voltage connector arrangement for certain electrical devices, such as surge arresters. Whilst surge arresters employing air gaps are known, surge arresters using varistor, and especially metal oxide varistor (MOV), blocks are commonly used. Typically such a surge arrester comprises a plurality of substantially solid cylindrical blocks of MOV material compressed in end-to-end relationship between a pair of cylindrical metal electrodes, all sealingly encased within an insulating housing, for example of silicone polymer. A conductive layer is then applied to the outer polymer surface of the housing to provide the required screening.
Referring to FIG. 1, a known high voltage connector arrangement comprises a T-shaped screened connector 2 and an elongate cylindrical surge arrester module 4. The connector 2 comprises an insulating housing 5 of silicone polymer that defines a transverse passageway 6, and a further passageway 8 extending at right angles thereto. The passageway 6 is terminated at one end by a flange 10 for mounting the connector 2 onto a bushing of switchgear (not shown). The other end of the passageway 6 is closed by a cap 12.
The surge arrester module 4 comprises a lower electrode 14, an upper electrode 16, and a plurality of varistor blocks 18, such as MOV blocks for example, extending end to end between the electrodes. The electrode and varistor block structure is held together longitudinally in compression (by means not shown) and is enclosed within silicone rubber insulation 20, with a lower terminal 22 protruding therefrom and a connecting lug 24 protruding from the upper electrode 16. It should be understood that while the insulation 20 is described here as being silicone rubber, other suitable materials having desirable insulating properties for a particular application may be used instead. The surge arrester module 4 is push-fitted into a connector arm 26 of the housing 5 of the connector 2 that contains the passageway 8, such that the connecting lug 24 projects into the passageway 6 and is secured therein to a metal plate 28 by a transverse bolt 30. The housing 5 is electrically screened by means of a conductive layer 32 on the outer surface thereof, which is connected to a terminating pigtail 34 for connection to an earth ground. The surge arrester module 4 is also screened by a conductive layer 36 that extends from the lower end thereof and terminates partway along the stack of varistor blocks 18 at its upper end termination 38 within the connector arm 26. The location within the connector arm 26 of the upper end termination 38 of the conductive layer 36 is typically 5 to 10 mm from the end of the connector arm 26, thus providing a working tolerance to ensure that the conductive layer 36 is enclosed by the connector arm 26.
A conductive layer 40 extends around the inside of the passageway 8 so as to enclose the connecting lug 24 of the surge arrester module 4 within a Faraday Cage.
In operation, the flange 10 of the connector 2 is mounted onto a bushing of the switchgear, thereby establishing an electrical connection via the bolt 30 to the surge arrester module 4.
However, it has been found that with this arrangement operating at high voltage, the electrical field at the upper end termination 38 of the conductive layer 36, within the screened insulated connector arm 26 of the connector 2, can be unacceptably high. A discontinuity in the electrical field distribution arises at the end of the conductive layer 36 within the connector, resulting in an unacceptably high electrical field at the interface between the surge arrester module 4 and the connector 2. Furthermore, it has been found that the short circuit current performance is poor, allowing a high current to flow between the upper electrode 16 and the lower electrode 14 through the varistor blocks 18. In the event of high current flowing through the arrangement, a resulting electric arc passes between one electrode 16 of the surge arrester module 4 to the other electrode 14 through the varistor blocks 18. Under these circumstances, at sufficiently high energy, the surge arrester module 4 can fail explosively and unacceptably.